Various aluminum alloy sheets are used in components (members or parts) of transportation equipment such as automobiles, ships, and aircraft as being appropriately selected according to their properties. Independently, weight reduction of the components has been demanded so as to provide better fuel efficiency by in consideration of global environmental issues such as CO2 emission control. This increases the use of aluminum alloy sheets, which have a specific gravity of about one-third that of iron and have excellent energy absorption.
For example, automotive components employ Mg-containing aluminum alloy sheets such as Japanese Industrial Standard (JIS) 5xxx-series Al—Mg alloy sheets and JIS 6xxx-series Al—Mg—Si alloy sheets. These aluminum alloy sheets are bonded (joined) by welding or adhesion with an adhesive, or both in combination. The bonding by welding bonds the aluminum alloy sheet at points or in a line, whereas the bonding by adhesion with an adhesive bonds the aluminum alloy sheet in the entire surface, thereby offers a high bond strength, and is advantageous typically in crashworthiness. The adhesion with an adhesive has been recently increasingly employed in automotive components.
Disadvantageously, however, such aluminum alloy automotive components bonded with an adhesive, when any of moisture, oxygen, and chloride ions permeates the bonded portion, have a lower bond strength because the interface between the adhesive layer and the aluminum alloy sheet gradually deteriorates to cause interfacial peeling. As possible solutions to this, there have been investigated techniques to prevent the bond strength reduction and to allow an aluminum alloy automotive component including an adhesive layer to have better bond durability (see, for example, Patent Literature 1 to 3).
Typically, Patent Literature 1 proposes a technique in which a Mg-enriched layer is removed from an aluminum alloy sheet surface, and Cu is enriched in the aluminum alloy sheet surface both by one acid wash treatment. Patent Literature 2 proposes a technique in which an aluminum alloy sheet having an OH absorption coefficient controlled within a specific range in relation to the amount of Mg enriched (segregated) in the surface of the sheet. Patent literature 3 proposes a technique in which a solution heat treatment and a hot-water treatment are successively performed so as to control a Mg content, Si content, and a OH content in an oxide film surface layer of an aluminum material within specific ranges.